esv.el: add esv-close-existing-buffers custom

esv.el

@@ -39,6 +39,10 @@
   :prefix "esv-"
   :group 'applications)
 
+(defcustom esv-close-existing-buffers nil
+  "Whether to close an existing `esv' buffer if one already exists."
+  :type 'boolean
+  :group 'esv)
 (defcustom esv-columns 72
   "Length of each line output by `esv'."
   :type 'natnum
@@ -78,8 +82,10 @@ esv can be acquired at <https://sr.ht/~jeremy/esv>."
                          (list (format "-l%d" esv-columns)))))
     (catch 'buffer-exists
       (when (get-buffer buffer)
-        (message "Buffer `%s' already exists" buffer)
-        (throw 'buffer-exists nil))
+        (unless esv-close-existing-buffers
+          (message "Buffer `%s' already exists" buffer)
+          (throw 'buffer-exists nil))
+        (kill-buffer buffer))
       (apply #'call-process esv-program nil buffer t
         (append arg-list (list book) (list verses)))
       (display-buffer buffer)